In a communications system which employs multiple access techniques such as frequency domain or code division multiplexing, several subscribers make use of a single communications resource. For example, in a frequency division multiplexed system, each subscriber unit is assigned a particular frequency for use while that subscriber is engaged in a call with another, similarly equipped, subscriber unit. When the subscriber has completed transmissions, the frequency resource can be reassigned for use by another subscriber. However, in a typical frequency domain system, a limited number of frequency channels are available for use by subscribers. When this capacity limit has been reached, the communication system cannot accommodate additional subscribers.
A code division multiplexed system has an advantage over a frequency domain system in that the code division multiplexed system embodies a more efficient use of the frequency spectrum allocated for the communication system. In a code division multiplexed system, each subscriber unit is assigned a unique pseudonoise code while that subscriber unit is engaged in a call. Each subscriber unit makes use of the same frequency spectrum as other subscriber units, but is assigned a different pseudonoise code. However, in a code division multiplexed system, elaborate hardware must be employed in order to acquire and track the pseudonoise codes used by the various subscriber units. Additionally, in order for the communication system to be available to a large number of subscriber units, a rigid timing structure must be used in order to synchronize the code generator of a transmitting station with the code generator of a receiving station. This additional overhead reduces the flexibility of the system as well as increases the costs associated with operating the communication system.
Therefore, what is needed are a method and apparatus for achieving spectrally efficient communications without requiring the strict timing structure required in a code division multiplexed system.